In my U.S. Pat. No. 3,904,839 granted Sept. 9, 1975, there was described a fault locator for telephone cable pairs employing a swept frequency test signal in the range of 10 kHz to 500 kHz by means of which the derivative with respect to frequency of the phase angle of the input impedance of a loop pair can be measured and applied to a spectrum analyzer to determine distances to impedance irregularities in proportion to the frequencies of observed power spectrum maxima. I have since discovered that the equivalent measurement can be obtained from the magnitude function of the input impedance that is advantageously more insensitive to measurement errors and noise on the transmission pair. Measurement sensitivity can further be advanced by passing the equivalent signal obtained from the magnitude function through a filter window of appropriate characteristic. In this way "ripples" in the waveform presented to a spectrum analyzer are enhanced with the result that the distances to more remote faults can be determined from a single test position.
Where my previous invention required the measurement of both voltage and current components of the input impedance of a loop under test, measurements of one parameter only (voltage or current) are required in the present invention.